The present invention relates to a metal thin-film type magnetic recording medium and a process for fabricating the same.
Ferromagnetic thin-film type magnetic recording media are prepared by forming a thin film of iron, cobalt, nickel or an alloy thereof by a vacuum deposition, sputtering or electroplating method over the surface of a high-molecular weight compound film. Such magnetic recording media have an advantage in comparison with over coated type magnetic recording media prepared by applying over the surface of a base or substrate a mixture of ferromagnetic particles and a binder in that the former have a far higher recording density than the latter. However, there are differences in hardness, mechanical strength and thermal properties between magnetic thin films and bases or substrates which are made of high-molecular weight compounds in the case of the fabrication of magnetic tapes or flexible magnetic disks. Therefore, during the fabrication process or in use, if the magnetic tapes or disks are subjected to forces or thermal stresses in excess of their permissible limits, magnetic thin films are easily cracked. Once cracks have propagated, head-contactability; that is, a degree of capability of a magnetic tape or disk to contact a magnetic recording-and-reproducing head, is degraded depending upon the configurations of cracks. As a result, wear of the magnetic head is accelerated and the thin film itself is rapidly damaged. In the case of the home video tape recorder, a magnetic tape is wrapped around a cylinder and rotary heads are used to record or reproduce video and audio signals. In the still-picture reproduction mode, cracks tend to develop in the direction in which the rotary heads rotate. These cracks adversely affect "still-picture-reproducibility-life; that is, a life of a magnetic film during which still pictures can be satisfactorily reproduced.
In general, polyester films have been widely used as bases or substrates for the production of thin-film type magnetic recording media. In the preparation of such films, their surfaces are formed with extremely fine projections of residues or catalytic particles or specially added finely divided particles so that the lubricity; that is, the property to lessen friction, of the film surfaces can be improved. When a thin film of a ferromagnetic metal or alloy is formed over the surface of such film, cracks tend to develop from the peaks of the projections. In the still-picture reproduction modes, the rotary magnetic heads tend to scratch the ferromagnetic thin film from its cracks and finally separate it from the base or substrate. As a result, the still-picture-reproducibility-life becomes very short. In addition, surface roughness cannot be reduced less than 0.1 .mu.m in Rmax. Rmax is defined in JIS B0601-1976 or ISO R468-1966. It is rated as a value in .mu.m obtained by dividing a maximum peak-to-valley distance over a reference length by a longitudinal magnification.
When thin films are formed over the smooth surfaces of polyester film bases or substrates free from such extremely fine projections as described above, friction between the cylinder of a rotary head assembly and a magnetic tape is so high that smooth transportation of the magnetic tape cannot be ensured.
The vacuum evaporation, ion-plating or sputtering method by which magnetic thin films are deposited in a vacuum is advantageous over the wet plating method in that uniform magnetic characteristics are ensured over a long length of magnetic recording media and no pollution problem arises from the disposal of wastes. However, when magnetic thin films are formed, film bases or substrates are subjected to heat in vacuum or at reduced pressures so that gases mainly consisting of water vapor and polyester oligomer evolve from the surfaces of the base or substrate. As a result, the magnetic and other characteristics of finished products are adversely affected. Water vapor can be removed by a pre-drying step, but it is difficult to eliminate polyester oligomers by a preliminary step.